Add new texel buffer type.

GLSL 3+ shaders can read from texel buffers by declaring a uniform samplerBuffer variable and calling texelFetch with a 0-based index, in the shader code. All attributes in a texel buffer's format array must have the same data format (e.g. all 'floatvec4' formats).

Shader:send accepts a texel Buffer for samplerBuffer uniforms.

Added 'texelbuffer' to love.graphics.getSupported (requires GLSL 3+ and desktop OpenGL).
Added 'texelbuffersize' to love.graphics.getSystemLimits. Returns the max number of values in a texel buffer (array length multiplied by the number of attributes declared in the buffer's format array).

src/modules/graphics/opengl/Buffer.h

@@ -53,7 +53,9 @@ public:
 	void unmap() override;
 	void setMappedRangeModified(size_t offset, size_t size) override;
 	void fill(size_t offset, size_t size, const void *data) override;
-	ptrdiff_t getHandle() const override;
+
+	ptrdiff_t getHandle() const override { return buffer; };
+	ptrdiff_t getTexelBufferHandle() const override { return texture; };
 
 	void copyTo(size_t offset, size_t size, love::graphics::Buffer *other, size_t otheroffset) override;
 
@@ -67,8 +69,11 @@ private:
 	BufferType mapType = BUFFERTYPE_VERTEX;
 	GLenum target = 0;
 
-	// The VBO identifier. Assigned by OpenGL.
-	GLuint vbo = 0;
+	// The buffer object identifier. Assigned by OpenGL.
+	GLuint buffer = 0;
+
+	// Used for Texel Buffer types.
+	GLuint texture = 0;
 
 	// A pointer to mapped memory.
 	char *memoryMap = nullptr;