opengl: simplify TexCoordsMaps in sampler

The short-term goal is simplify the code (both C and GLSL).

But in the end, I'd like to modify the OpenGL filters API to expose the input textures to the filters, and let them compute the texture coordinates in CPU (so that the matrix multiplications are not executed from the fragment shader). By making the coordinates transformation identical for all planes, this will allow to simplify the API.

---

The input picture may contain padding, for two reasons:

1. the rectangle defined by the picture_t offset and visible width and height may not cover the whole picture size;
2. if the OpenGL API does not support NPOT textures, textures are created with power-of-two dimensions.

As a consequence, for sampling, the coordinates (between 0 and 1 in both dimensions) must be transformed to take the padding into account.

These transformations were computed and performed per-plane. Therefore, there were n "TexCoordsMaps" matrices for a picture with n planes.

However, these transformations are necessarily the same for all planes:

1. the picture_t offset/sizes are defined for the whole picture (then scaled appropriately for each plane);
2. since the ratio between different planes of the same pictures is always a power of two (e.g. in YUV 4:2:0, the size of U and V planes are half the size of the Y plane in both dimensions), this property holds even if we align the dimensions of each plane texture to the next power-of-two.

Therefore, use a single transformation matrix.

---

Concretely, before the change, the TexCoordsMaps were applied separately for each plane (this is an fragment shader with an YUV 4:2:0 input):

uniform mat4 TransformMatrix;
uniform mat4 OrientationMatrix;
uniform sampler2D Textures[3];
uniform mat3 TexCoordsMaps[3];

uniform mat4 ConvMatrix;
vec4 vlc_texture(vec2 pic_coords) {
 vec3 pic_hcoords = vec3((TransformMatrix * OrientationMatrix * vec4(pic_coords, 0.0, 1.0)).st, 1.0);
 vec2 tex_coords;
 vec4 texel;
 vec4 pixel = vec4(0.0, 0.0, 0.0, 1.0);
 tex_coords = (TexCoordsMaps[0] * pic_hcoords).st;
 texel = texture2D(Textures[0], tex_coords);
 pixel[0] = texel.r;
 tex_coords = (TexCoordsMaps[1] * pic_hcoords).st;
 texel = texture2D(Textures[1], tex_coords);
 pixel[1] = texel.r;
 tex_coords = (TexCoordsMaps[2] * pic_hcoords).st;
 texel = texture2D(Textures[2], tex_coords);
 pixel[2] = texel.r;
 vec4 result = ConvMatrix * pixel;
 result = _main_0_0(result);
 return result;
}

After this change, it is applied once for the whole picture:

uniform mat4 TransformMatrix;
uniform mat4 OrientationMatrix;
uniform sampler2D Textures[3];
uniform mat3 TexCoordsMap;

uniform mat4 ConvMatrix;
vec4 vlc_texture(vec2 pic_coords) {
 vec2 tex_coords = (TexCoordsMap * (TransformMatrix * OrientationMatrix * vec4(pic_coords, 0.0, 1.0)).xyz).xy;
 vec4 texel;
 vec4 pixel = vec4(0.0, 0.0, 0.0, 1.0);
 texel = texture2D(Textures[0], tex_coords);
 pixel[0] = texel.r;
 texel = texture2D(Textures[1], tex_coords);
 pixel[1] = texel.r;
 texel = texture2D(Textures[2], tex_coords);
 pixel[2] = texel.r;
 vec4 result = ConvMatrix * pixel;
 return result;
}